Has anyone encountered rust on SLR Mounting rim? Not those pins or marks, but the metallic rim which is to match with SLR lenses.

I suppose both mounting rims from body and lense are of same material. However, mine has slight rust stains.
I went for servicing the camera, and the technician said that it will rust if contact with water, like raindrop.

Could anyone advice if this reply is reasonable. And what could be the material of the rim of my Canon 70D.

Although is steel, the mount can rust and you will notice that for the screws which have the paint scratched. This rusting process is amplified if the camera was in contact with sea water (salt + water). Even the water vapor in the atmosphere at the sea side may create such an effect.
Can you confirm to us that you did a few shots at the seaside and did not clean the Lens hood with a damp cloth to remove the salt? Your confirmation may help others.

Aluminium doesn't rust in the way we'd associate with iron - the orange flakey stuff. It does have an oxidation layer, but that is simply the dull gray layer that you see on any outside surface of the aluminium and it actually protects the aluminium inside. Aluminium can't "rust" as such.

Steel? Yes

Steel is very susceptible to rusting. Rust will turn orange/brown, puff up and flake off and weaken the metal. This is accelerated by water which is why we associate rust with water but air alone will rust it.

Stainless steel? Yes, but it's somewhat resistant to rusting.

Stainless steel is capable of rusting just like steel, but the Chromium content makes it quite resistive to rusting. Chromium has an oxidation layer like Aluminium which protects the metal inside if the Chromium content is high enough. Stainless steel used in kitchens and cutlery has enough of it to virtually ensure against rusting in normal circumstances; other types of stainless steel such as on tools or mechanical parts may be a little bit more susceptible to rust if it uses a lower grade.

Painted steel? Yes, if the paint is chipped or scratched off anywhere.

The paint/primer will protect the steel to some extent but scratches or chips that reveal the steel will allow it to rust.

Galvanised steel/iron? Yes, if the zinc layer is scraped off.

The zinc layer prevents rust as long as it's not chipped or scraped off. This zinc layer is usually a little bit more hard-wearing than paint would be but usually makes it non-smooth.

Brass? Not really.

Brass can corrode, but is fairly resistant to it. It's also rarely used pure, it's usually an alloy with other things and when it's an alloy with aluminium it's corrosion-proof enough to use it on things like plumbing or on ships.